In my U.S. Pat. Nos. 4,030,895 and 4,095,958, there are shown and described retort apparatus designed to generate combustible fuel gases from biomass material. In each patented apparatus, there is a fixed, downwardly-inclined grate supported within an enclosure defining a chamber and a grate dividing the chamber into a plenum chamber beneath the grate and a combustion chamber above the grate. The biomass material is fed to the upwardly-facing side of the grate at the upper end and the ash is removed from the upwardly-facing side of the grate at the lower end. The grate in U.S. Pat. No. 4,040,895 is comprised of spaced, parallel grate bars arranged transversely of the chamber and the biomass moves gravitationally downwardly over the surface thereof. The grate in U.S. Pat. No. 4,095,958 is also stationary and comprises an inclined flat plate containing a plurality of holes. The biomass is fed to the upper end of the plate and slides downwardly thereon by gravity. In the latter apparatus, longitudinally-spaced, parallel fins at the underside of the grate were provided in order to keep the grate as cool as possible and thereby reduce the formation of slag. It is the purpose of this invention to provide a retort with an improved grate system with a moving grate and under grate air distribution which permits operation over a wide range of firing rates while maintaining a low grate temperature well within a reasonable working range of available alloys suitable for grate construction, hence, substantially lengthening the working life of the grate and reducing the operating temperature of the under grate structure, thus relieving thermal stress and reducing heat loss.